This invention relates to pneumatic deicers of the type in which a boot of resilient material such as rubber is attached to the leading edge of an airfoil and has inflatable passages which are alternately extended by inflation pressure to break up ice accumulation which tends to form on the surface of the airfoil. One of the problems with deicers used heretofore, especially on aircraft which travel at relatively slow speeds, has been the difficulty in cracking the ice cap which builds up at the leading edge. This ice cap in some cases has remained substantially intact and has been moved outwardly as a unit when the passages have been inflated rather than being cracked to facilitate total removal of the ice.